A first form of this invention relates to a method of controlling an anti-lock brake system for vehicles which prevents the locking of the wheels upon emergency braking of a vehicle, the method being characterized in that it uses the road surface friction force F or road surface friction coefficient μ, instead of slip factor used in the prior art, to control the system.
A second form of this invention relates to a method of finding a control point in an anti-lock brake system (ABS) for vehicles which prevents the locking of the wheels upon emergency braking of a vehicle, the method makes an errorless ABS control decision by using a road surface friction force detecting sensor or a road surface friction coefficient detecting sensor.
Method of Controlling Anti-Lock Brake System for Vehicles
Generally, conventional anti-lock brake systems for vehicles, e.g., automobiles, automatically control the brake operation such that the slip ratio falls in a given range on the basis of the vehicle speed and wheel speed (e.g., Japanese Patent Publication No. 30585/1984 and Japanese Patent Application Laid-Open Specification No. 61354/1985). The relation between road surface friction coefficient μ, and slip ratio can vary depending on the road surface conditions and for this reason the systems sometimes fail to provide a maximum brake pressure, in which case a minimum brake distance cannot be obtained. Further, since the vehicle speed is a value estimated from the wheel speed, there is a problem involving precision in the control of slip ratio. To accurately know the vehicle speed, there is a need for a complicated device, such as a ground-relative speed sensor (e.g., Japanese Patent Application Laid-Open Specification No. 64861/1988) or a vehicle deceleration sensor (e.g., Japanese Patent Application Laid-Open Specification No. 170157/1988). In the device described in Japanese Patent Application Laid-Open Specification No. 25169/1988, the torque of road surface friction force (tire torque) acting on the wheel is found by calculation from wheel angular acceleration and brake liquid pressure and that value of the tire torque at which the tire torque starts to decrease during the increase of the brake liquid pressure is employed as one of the factors for deciding the conditions immediately before the locking of the wheel. However, in this device, the tire torque is indirectly found by calculation from the wheel angular acceleration and brake liquid pressure and on account of the presence of uncertain constants such as brake efficiency and the moment of inertia of the wheel, there is a problem on precision. Further, since the pneumatic pressure in the tire of the wheel and the distance from the ground to the vehicle vary, there is also a problem that the ratio between the road surface friction force and the tire torque is not always maintained at a constant value.
In order to eliminate the drawback inherent in the conventional device described above, the present applicant has previously proposed, in Japanese Patent Application No. 197809/1989 (Japanese Patent Application Laid-open Specification No. 220056/1991), an anti-lock brake system for vehicles, comprising a strain gauge disposed in the vicinity of the axle, a load surface friction force detecting device having means for directly measuring shearing strains in the vicinity of the axle, and a vertical load detecting device, and means whereby in response to an output signal from a road surface friction coefficient detecting device having means for arithmetically processing detection signals from the two devices, the brake pressure is increased when the road surface friction force or road surface friction coefficient increases with increasing brake pressure or it is decreased when the road surface friction force or road surface friction coefficient decreases despite increasing brake pressure, and if the road surface friction force or road surface friction coefficient decreases with decreasing brake pressure, the brake pressure is increased again, such operations being repeated.
In the case where anti-lock brake control for vehicles is effected by using the system, it has been found that owing to disturbance sources such as vibrations of the tire and road surface during brake operation and the suspension, the signal value of the road surface friction force F or road surface friction coefficient μ sometimes fluctuates in a certain range, causing the accurate control start point to be mistaken.
Method of Detecting Control Point in ABS
Generally, conventional anti-lock brake systems for vehicles, e.g., automobiles, automatically control the brake operation such that the slip ratio falls in a given range on the basis of the vehicle speed and wheel speed (e.g., Japanese Patent Publication No. 30585/1984 and Japanese Patent Application Laid-Open Specification No. 61354/1985). The relation between road surface friction coefficient and slip ratio can vary depending on the road surface conditions and for this reason the systems sometimes fail to provide a maximum brake pressure, in which case a minimum brake distance cannot be obtained. Further, since the vehicle speed is a value estimated from the wheel speed, there is a problem on precision in the control of slip ratio. To accurately know the vehicle speed, there is a need for a complicated device, such as a ground-relative speed sensor (e.g., Japanese Patent Application Laid-Open Specification No. 64861/1988) or a vehicle deceleration sensor (e.g., Japanese Patent Application Laid-Open Specification No. 170157/1988). In the device described in Japanese Patent Application Laid-Open Specification No. 25169/1988, the torque of road surface friction force (tire torque) acting on the wheel is found by calculation from wheel angular acceleration and brake liquid pressure and that value of the tire torque at which the tire torque starts to decrease during the increase of the brake liquid pressure is employed as one of the factors for deciding the conditions immediately before the locking of the wheel. However, in this device, the tire torque is indirectly found by calculation from the wheel angular acceleration and brake liquid pressure and on account of the presence of uncertain constants such as brake efficiency and the moment of inertia of the wheel, there is a problem on precision. Further, since the pneumatic pressure in the tire of the wheel and the distance from the ground to the vehicle vary, there is also a problem that the ratio between the road surface friction force and the tire torque is not always maintained at a constant value.
In order to eliminate the drawback inherent in the conventional device described above, the present applicant has previously proposed, in Japanese Patent Application No. 197809/1989 (Japanese Patent Application Laid-open Specification No. 220056/1991), an anti-lock brake system for vehicles, comprising a strain gauge disposed in the vicinity of the axle, a load surface friction force detecting device having means for directly measuring shearing strains in the vicinity of the axle, and a vertical load detecting device, and means whereby in response to an output signal from a road surface friction coefficient detecting device having means for arithmetically processing detection signals from the two devices, the brake pressure is increased when the road surface friction force or road surface friction coefficient increases with increasing brake pressure or it is decreased when the road surface friction force or road surface friction coefficient decreases despite increasing brake pressure, and if the road surface friction force or road surface friction coefficient decreases with decreasing brake pressure, the brake pressure is increased again, such operations being repeated.
In the case where anti-lock brake control for vehicles is effected by using the above system, it has been found that owing to crosstalk such as brake torque contained in sensor signals, the accurate control start point can often be mistaken.